The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a dimming control system for electronic ballasts.
Conventional dimming ballasts for gas discharge lamps include low voltage dimming circuitry that is intended to work in conjunction with an external dimming controller. The external dimming controller is connected to special inputs on the ballast via dedicated low voltage control wiring that, for safety reasons, cannot be routed in the same conduit as the AC power wiring. The external dimming controller is usually very expensive. Moreover, installation of low voltage control wiring is quite labor-intensive (and thus costly), especially in xe2x80x9cretrofitxe2x80x9d applications. Because of these disadvantages, considerable efforts have been directed to developing control circuits that can be inserted in series with the AC line, between the AC source and the ballast(s), thereby avoiding the need for additional dimming control wires. The resulting approaches are sometimes broadly referred to as xe2x80x9cline controlxe2x80x9d dimming.
A number of line control dimming approaches exist in the prior art. One known type of line control dimming approach involves introducing a notch (i.e., dead-time) into each and every cycle of the AC voltage waveform at or near its zero crossings. This approach requires a switching device, such as a triac, in order to create the notch. Inside of the ballast(s), a control circuit measures the time duration of the notch and generates a corresponding dimming control signal for varying the light level produced by the ballast. In practice, these approaches have a number of drawbacks in cost and performance. A significant amount of power is dissipated in the switching device, particularly when multiple ballasts are to be controlled. Further, the method itself distorts the line current, resulting in poor power factor and high harmonic distortion, and sometimes produces excessive electromagnetic interference. Additionally, the control circuitry tends to be quite complex and expensive.
An attractive alternative approach that avoids the aforementioned drawbacks is described in copending application Ser. No. 09/966,911, filed Sep. 28, 2001 and entitled xe2x80x9cDimming Control System for Electronic Ballastsxe2x80x9d which is assigned to the same assignee as the present invention. The circuitry detailed therein employs a wall-switch assembly comprising two switches and two diodes, and sends a dimming command by removing one or more positive half-cycles (corresponding to a xe2x80x9cdimxe2x80x9d command) or negative half-cycles (corresponding to a xe2x80x9cbrightenxe2x80x9d command) from the AC voltage supplied to the ballast. While this approach has a number of substantial benefits over prior systems, it is not ideally suited for those ballasts that include a boost converter front-end. More specifically, because the ballasts receive only one half of the AC line cycle during a light level change, the boost converter may undesirably fall out of regulation during those times. In order prevent this problem, one would have to design the boost converter to remain in regulation down to very low levels of AC line voltage (e.g., down to about 66% of the nominal AC line voltage), which would add significant cost to the ballasts.
What is needed, therefore, is a structurally efficient and cost-effective dimming control system that avoids any need for additional dimming control wires, but that does so without introducing undesirable levels of steady-state power dissipation, line current distortion, and electromagnetic interference, and without requiring that the ballasts remain in regulation down to very low levels of AC line voltage. A need also exists for a dimming control system that is structurally efficient and cost-effective. A dimming control system with these features would represent a significant advance over the prior art.